The subject matter of the present invention relates generally to metal detectors of the transmit-receive type which can be employed to detect buried metal objects in the presence of mineral soil or undesired objects. The present invention is an improvement on the sampling metal detector shown in U.S. Pat. No. 4,030,026 of George C. Payne granted June 14, 1977.
By employing a wide gating pulse having a pulse width greater than about 1/3 cycle and preferably about 1/2 cycle of the oscillator signal applied to the transmit coil, the present detector is provided with improved noise rejection. In addition, the present metal detector is provided with greater sensitivity and signal-to-noise ratio than such prior metal detector by employing balanced demodulation whereby both halves of one cycle of the received signal are averaged to produce the output voltage of the gating circuit corresponding to the detected metal object.
The above-identified patent discloses a sampling metal detector which eliminates the mineral soil signal component of the received signal by sampling such received signal when such mineral soil signal component is zero or of minimum amplitude. This minimum amplitude portion of the mineral soil signal component corresponds to when the reactive component of the received signal crosses the zero axis of such received signal. While in most cases this sampling technique is entirely satisfactory and enables the detection of a metal object in the presence of mineral soil, it is possible that in some cases inaccuracies may result due to the presence of noise signal components such as random high frequency noise or 60 cycle harmonics near the second and third harmonics of the oscillator signal occurring in the received signal at the portion which is gated by the sampling pulse.
It has been found that this noise problem can be avoided by using a wide sampling pulse having a width greater than about 1/3 cycle and preferably 1/2 cycle of the oscillator signal, and centering such sampling pulse on the received signal waveform at a centering position where the mineral soil signal component is of minimum amplitude. As a result, equal positive and negative polarity portions of the mineral soil signal component on opposite sides of the centering position, are transmitted through the gate means for each gating pulse to an averaging means where they are averaged to zero. The noise signal component also averages to zero in the averaging means which has an RC time constant about fifty times greater than the width of the gating pulse. This improved noise rejection is also possible when the metal detector is operated in a discriminate mode to discriminate the hunted object from undesired metal objects of the same type which produce a background signal component in the received signal, such background signal component also being sampled or gated at a position of minimum amplitude.
In addition, a metal detector of the present invention is provided with greater sensitivity and greater signal-to-noise ratio by employing balanced demodulation in the gating circuit. This balanced demodulation technique averages the gated portion of the received signal during one half cycle and averages the other one-half cycle portion of the received signal after inverting it in order to produce a higher average output voltage corresponding to the detected signal of the buried metal object.